1. Field of the Invention
The present invention relates to current control circuits, and in particular, to bidirectional current control circuits suitable for use in controlling the charging and discharging of rechargeable battery cells.
2. Description of the Related Art
Circuits for controlling the bidirectional flow of current can be found in many applications. One increasingly common application is that of current control circuits for controlling the charging and discharging of rechargeable battery cells, including lithium and lithium-ion battery cells such as those used in portable computers.
As is well known, lithium and lithium-ion secondary (rechargeable) battery cells require protection from prolonged over-charging, over-discharging and over-current to prevent degradation of the performance of the cells, as well as to prevent the possibility of rupturing the case of the cells and potential combustion of the electrolyte within the cells. Power metal oxide semiconductor field effect transistor (MOSFET) switches in series with the current path for the cells are commonly used to disconnect the cells from chargers that fail to terminate the charging operation within specified voltage limits. Such MOSFET switches are typically driven from protection circuits internal to the battery pack which monitor the individual cell voltages for over-charge and over-discharge, as well as monitor the current for the battery pack for over-current in either direction.
In many applications, there are actually two power MOSFETs that are serially connected with the cells. Each MOSFET switch is turned on or off depending upon the type of failure. Normally, both MOSFETs are turned on and when a fault occurs, one of the MOSFETs is turned off by the protection circuit. The MOSFET that is turned off blocks current flow in the battery pack in the same direction that the current was flowing when the fault occurred.
Normally, recovery of such protection circuits requires some form of external circuitry for monitoring the operating environment of the battery pack (e.g., temperature, etc.) or the terminal voltages on the protection MOSFETs. It would be. desirable, however, to avoid the need for such external circuitry and provide for self-recovery of the protection circuit following a return to a safe operating condition.